Josephus Baxter was so excited by the receipt of the letter which Koku delivered to him that for some seconds Tom Swift could get nothing out of him except the statement:

"I'm on their trail! Now I'm on their trail!"

"What do you mean?" Tom insisted. "Whose trail? What's it all about?"

"It's about Field and Melling! That's who it's about!" exclaimed Mr. Baxter, with a smothered exclamation. "Look, Tom Swift, this letter is addressed to me from one of the biggest dye firms in the world--a firm that is always looking for something new!"

"But if you haven't anything new to give them, of what use is it?" Tom asked, for he knew that the chemist had said his process, stolen, as he claimed, by Field and Melling, was his only new project.

"But I will have something new when I get those secret formulae away from those scoundrels!" declared Mr. Baxter.

"Yes, but how are you going to do it, when you can't even prove that they have them?" asked Tom.

"Ah, that's the point! Now I think I can prove it," declared Mr. Baxter. "Look, Tom Swift! This letter is addressed to me in care of Field and Melling at the office I used to have in their fireworks factory."

"The office from which you were rescued nearly dead," Tom added.

"Exactly. The place where you saved me from a terrible death. Well, if you will notice, this letter was written only two days ago. And it is the first mail I have received as having been forwarded from that address since the fire. I know other mail must have come for me, though."

"What became of it?" asked Tom.

"Those scoundrels confiscated it!" declared the chemist. "But, in some manner, perhaps through the error of a new clerk, this letter was remailed to me here, and now I have it. It is of the utmost importance!"

"In what way?" asked Tom.

"Why, it is directed to me, outside and in, and it makes an inquiry about the very dyes of the lost secret formulae, one dye in particular."

"I don't quite understand yet," said Tom.

"Well, it's this way," went on Mr. Baxter. "I had, in the office of Field and Melling, all the papers telling exactly how to make the dyes. After the fire, in which I was rendered unconscious, those papers disappeared.

"The only way in which any one could make the dyes in question was by following the formulae given in those papers. And now here is a letter, addressed to me from a big firm, asking my prices on a certain dye, which can only be made by the process bequeathed to me by the Frenchman."

"Which means what?" asked Tom.

"It means that Field and Melling must have been writing to this firm on their own hook, offering to sell them some of this dye. But, in some way, my name must have appeared on the letter or papers sent on by the scoundrels, and this big firm replies to me direct, instead of to Field and Melling! Even then I would not have benefited if they had confiscated this letter as I am sure, they have done in the case of others. But, by some slip, I get this.

"And it proves, Tom Swift, that Field and Melling are in possession of my dye formulae, and that they have tried to dispose of some of the dye to this firm. Not knowing anything of this, the firm replies to me. So now I have direct evidence--just what I wanted--and I can get on the trail of the scoundrels who have cheated me of my rights."

Tom looked at the letter which, it appeared, had been left with Koku by a special delivery boy from the post office. It was an inquiry about certain dyes, and was addressed to Mr. Baxter in care of Field and Melling, the former fireworks firm, which now had started a big dye plant, with offices in the Landmark Building in Newmarket.

"It does look as though you might get at them through this," Tom said, as he handed back the letter. "But I'm afraid you'll have to get further evidence before you could convict them in a court of law--you'll have to show that they actually have possession of your formulae."

"That's what I wish I could do," said the chemist, somewhat wistfully. His first enthusiasm had been lessened.

"I'll help you all I can," offered Tom. And events were soon to transpire by which the young inventor was to render help to the chemist in a most sensational manner.

"Just now," Tom went on, "I must arrange about getting a large supply of these chemicals made, and then plan for a test in some big city."

"Yes, you have done enough for me," said Mr. Baxter. "But I think now, with this letter as evidence, we'll be able to make a start."

"I agree with you," Tom said. "Why don't you go over to see Mr. Damon? He's a good business man, and perhaps he can advise you. You might also call on that lawyer who does work for Mr. Keith and Mr. Blake. And that reminds me I must call Mary Nestor up and find out when she is coming home. I promised to fetch her in one of the airships."

"I will go and see Mr. Damon," decided Mr. Baxter. "He always gives good advice."

"Even if he does bless everything he sees!" laughed Tom. "But if you're going to see him I'll run you over. I'm going to Waterfield."

"Thanks, I'll be glad to go with you," said the chemist.

Mr. Damon was glad to see his friends, and, when he had listened to the latest developments, he exclaimed with unusual emphasis:

"Bless my law books, Mr. Baxter! but I do believe you're on the right trail at last. Come in, and we'll talk this over."

So Tom left them, traveling on to a distant city where he arranged for a large supply of the chemicals he would need in his extinguisher.

For several days Tom was so busy that he had little time to devote to Mr. Baxter, or even to see him. He learned, however, that the chemist and Mr. Damon were in frequent consultation, and the young inventor hoped something would come of it.

Tom's own plans were going well. He had let several large cities know that he had something new in the way of a fire- fighting machine, and he received several offers to demonstrate it.

He closed with one of these, some distance off, and agreed to fly over in his aircraft and extinguish a fire which was to be started in an old building which had been condemned. and was to be destroyed. This was in a city some four hundred miles away and when Ned Newton called on him one afternoon he found Tom busily engaged in loading his sky-craft with a heavy cargo of the newest liquid extinguisher.

"You aren't taking any chances, are you, Tom?" asked Ned.

"What do you mean?"

"I mean you seem to have enough of the liquid 'fire- discourager' to douse any blaze that was ever started."

"No use sending a boy on a man's errand," said Tom. "I'm counting on you to go with me, Ned--you and Mr. Baxter. We leave this afternoon for Denton."

"I'll be with you. Couldn't pass up a chance like that. But here comes Koku, and it looks as if he had something on his mind."

The giant did, indeed, seem to be laboring under the stress of some emotion.

"Oh, Master Tom!" the big man exclaimed when he had got the attention of the young inventor. "Rad--he--he--"

"Has anything happened?" asked Tom, quickly. "No, not yet. But dat pill man--he say by tomorrow he know if Rad ever will see sunshine more!"

"Oh, the doctor says he'll be able to decide about Rad's eyesight tomorrow, does he?"

"Yes. What so pill man say," repeated Koku.

"Um," mused Tom, "I wish I were going to be here, but I don't see how I can. I must give this test." But it was with a sinking heart as he thought of poor Eradicate that the young inventor proceeded to pile into his airship the largest and heaviest load of chemicals it had ever carried.